The present invention relates to a process for the preparation of water-soluble polymers containing polyalkylene glycol ether side chains and having molecular weights of from 15,000 to 60,000 by solution polymerization of esters of ethylenically unsaturated carboxylic acids and polyalkylene glycols which are masked on the end groups at one end, if desired together with other monomers in at least one hydrocarbon in the presence of free-radical-forming polymerization initiators and, if desired, polymerization regulators at temperatures of up to 200xc2x0 C., removal of the solvent after the polymerization, and dissolution of the polymers in water to give from 50 to 80% strength by weight solutions, with polymers containing acid groups being neutralized if desired.
DE-A-19 653 524 describes the preparation of copolymers of methylpolyethylene glycol methacrylates and methacrylic acid by polymerization in aqueous medium using water-soluble initiators and water-soluble regulators. The concentrations in the polymerization are about 20% by weight. If the solids concentration in the polymer is increased, polymers having very high polydispersity and a high proportion of high-molecular-weight polymers are formed. Such products are ineffective as dispersants.
Esters of ethylenically unsaturated carboxylic acids and polyalkylene glycols which are masked on the end groups at one end may also be prepared by bulk polymerization or by solution polymerization in an aliphatic or aromatic hydrocarbon, cf. EP-A-753 488. However, polymerization in an aqueous medium, which gives about 25% strength by weight aqueous polymer solutions, is preferred since particularly effective dispersants containing polyalkylene glycol ether side chains are obtained. If the polymerization of the esters of ethylenically unsaturated carboxylic acids and polyalkylene glycols which are masked on the end groups at one end is carried out at high concentration, for example at concentrations of greater than 50% by weight, products which have only low efficacy as dispersants are obtained.
It is an object of the present invention to provide a process for the preparation of highly concentrated, but still readily flowable aqueous solutions of water-soluble polymers which contain polyalkylene glycol ether side chains and which are effective as dispersants for finely divided inorganic substances.
We have found that this object is achieved by a process for the preparation of water-soluble polymers containing polyalkylene glycol ether side chains and having molecular weights of from 15,000 to 65,000 by solution polymerization of esters of ethylenically unsaturated carboxylic acids and polyalkylene glycols which are masked on the end groups at one end, if desired together with other monomers in at least one hydrocarbon in the presence of free-radical-forming polymerization initiators and, if desired, polymerization regulators at temperatures of up to 200xc2x0 C., removal of the solvent after the polymerization, and dissolution of the polymers in water to give from 50 to 80% strength by weight solutions, with polymers containing acid groups being neutralized if desired, if the concentration of the monomers in the hydrocarbons is from 70 to 95% by weight. The polymerization is preferably carried out at a concentration of the monomers of from 80 to 90% by weight.
Preferably,
(a) esters of the formula 
in which
R1 and R2 are identical or different and are H or CH3,
A is an alkylene group having from 2 to 4 carbon atoms or is xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94,
is C1- to C50-alkyl or C1- to C18-alkylphenyl, and
is a number from 2 to 300,
are copolymerized with
(b) at least one monoethylenically unsaturated carboxylic acid or salts thereof. The monomers (a) and (b) are usually copolymerized in a weight ratio of from 98:2 to 2:98.
Particular preference is given to the copolymerization of
(a) acrylic acid esters or methacrylic acid esters of polyalkylene glycols which are masked on the end groups at one end by C1- to C4-alkyl and have molecular weights of from 200 to 10,000
with
(b) acrylic acid and/or methacrylic acid.
Particularly effective dispersants are obtained if
(a) esters of methacrylic acid and methylpolyethylene glycol having molecular weights of from 350 to 10,000
are copolymerized with
(b) methacrylic acid.
According to a preferred embodiment of the process according to the invention, the copolymerization is carried out in the presence of from 0.1 to 10% by weight of a polymerization regulator. Examples of suitable polymerization regulators are sodium hydrogensulfite, sodium bisulfite, sodium thiosulfate, sodium hypophosphite, phosphorous acid, 2-mercaptoethanol, dodecyl mercaptan, mercaptopropionic acid, mercaptoacetic acid, alkali metal salts of the said acids or mixtures of the polymerization regulators.
The alkylpolyalkylene glycols used can be, for example, compounds of the general formulae
R1xe2x80x94(Oxe2x80x94CHR2xe2x80x94CHR3)nxe2x80x94OH or R1xe2x80x94(Oxe2x80x94CH2xe2x80x94CH2xe2x80x94CH2)nxe2x80x94OH
where
R1=C1- to C50-alkyl
R2 and R3=H, methyl or ethyl,
n=from 2 to 300.
The molecular weight of the alkylpolyalkylene glycols can be up to 10,000, with a molecular weight of from 200 to 2000 being preferred. This corresponds to up to 230, preferably from 3 to 40, alkylene oxide units per molecule.
Examples of alkylpolyalkylene glycols are methylpolyethylene glycols having molecular weights of 350, 500, 750, 1000, 1500, 2000, 4000 and 10,000.
The alkylpolyalkylene glycols may also contain propylene oxide or butylene oxide units in combination with ethylene oxide units. The alkylene oxide units may be arranged in block form or randomly.
Examples thereof are methylpolyalkylene glycols which are obtainable by adduction of 5 mol of ethylene oxide and 1 mol of propylene oxide, 5 mol of ethylene oxide and 3 mol of propylene oxide, 5 mol of ethylene oxide and 10 mol of propylene oxide and 10 mol of ethylene oxide and 1 mol of propylene oxide, 10 mol of ethylene oxide and 3 mol of propylene oxide, 10 mol of ethylene oxide and 10 mol of propylene oxide, 20 mol of ethylene oxide and 1 mol of propylene oxide, 20 mol of ethylene oxide and 3 mol of propylene oxide, 20 mol of ethylene oxide and 10 mol of propylene oxide, 25 mol of ethylene oxide and 1 mol of propylene oxide, 25 mol of ethylene oxide and 3 mol of propylene oxide and 25 mol of ethylene oxide and 10 mol of propylene oxide ontoin each case 1 mol of methanol.
The polyalkylene glycols which are masked on the end groups at one end may also be polytetrahydrofurans having molecular weights of, for example, from 200 to 10,000 which carry a C1- to C4-alkyl group as end group.
The unsaturated carboxylic acids employed are preferably acrylic acid, methacrylic acid, maleic acid and maleic anhydride.
Esters of alkylpolyalkylene glycols and unsaturated carboxylic acids can be prepared, for example, by acid-catalyzed esterification of unsaturated carboxylic acids with alkylpolyalkylene glycols by all known processes. The water of reaction can remain in the mixture or be removed from the reaction mixture by, for example, azeotropic distillation using an entraining agent. Thus, the conversion of the alcohol component can be up to 100%, cf. EP-A-0 884 290.
In the esterification, the molar ratio of carboxylic acid to alkylpolyalkylene glycol can be, for example, from 10:1 up to 1:1.
Suitable entraining agents are all organic solvents having a boiling point of from 50 to 300xc2x0 C. The proportion of entraining agent is, for example, from 5 to 20% by weight, based on the sum of the carboxylic acid and alcohol components. Examples of entraining agents are n-paraffins, such as hexane, decane, undecane, dodecane, octadecane, isoparaffins, such as isooctane, isononane, isodecane, isododecane, isohexadecane, isooctadecane, cycloparaffins, such as cyclohexane, methylcyclohexane, dimethylcyclohexane, aromatics, such as benzene, toluene, o-, m- and p-xylene, xylene mixtures, trimethylbenzene, tetramethylbenzene, mesitylene, ethylbenzene, isopropylbenzene, n-butylbenzene and isobutylbenzene. Preference is given to cyclohexane, toluene, xylene mixtures and o-xylene. Particular preference is given to cyclohexane, methylcyclohexane, toluene and xylene.
It is also possible to use technical-grade mixtures of different entraining agents. Such mixtures often result in a boiling range. Such mixtures are also known as special boiling-point gasoline, petroleum spirit, special boiling-point spirit, naphtha or petroleum ether fractions. Such mixtures often arise as refinery fractions. They can be produced specifically from steam-cracker olefins by oligomerizations and hydrogenations.
Examples of special boiling-point gasolines of this type are gasolines having boiling ranges of from 90xc2x0 C. to 100xc2x0 C., from 100xc2x0 C. to 140xc2x0 C. or from 140xc2x0 C. to 160xc2x0 C. They are usually mixtures of alkanes. Fractions may comprise purely n-aliphatic, isoaliphatic, aliphatic-aromatic or purely aromatic constituents, depending on the origin.
Suitable entraining agents are all hydrocarbons or hydrocarbon mixtures. An overview of the hydrocarbon mixtures available industrially is given, for example, in Kirk-Othmer, Encyclopedia of Chemical Technology, 1995, Vol. 13, pages 744 ff., Hydrocarbons chapter or ebenda Vol. 12, pages 126 ff., Fuels chapter and Vol. 12, pages 341 ff., Gasoline chapter. A further overview of hydrocarbon mixtures is given in ullmann""s Encyclopedia of Industrial Chemistry, 1989 Vol. A 13, pages 227-281 in the Hydrocarbons chapter, as well as ebenda in Vol. A 16, pages 719-755 in the Motor Fuels chapter.
The entraining agents form, together with water, an azeotropic mixture, which generally has a boiling point below that of the low-boiling constituent. The boiling points of the azeotropic mixtures are, for example, particularly preferably from 70xc2x0 C. to 130xc2x0 C.
The esters can also be prepared by alkali-catalyzed transesterification of unsaturated carboxylic acid esters with alkylpolyalkylene glycols, cf. WO-A-97/26293.
Alkylene glycol monomethacrylates or monoacrylates can also be prepared, for example, as described in WO-A-99/10407 by alkoxylation of acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate or hydroxypropyl acrylate by means of complex-bound cyanides as catalysts.
The initiators used for the polymerization are initiators which are soluble in the polymerization medium. Examples are organic peroxides and azo initiators, for example diacyl peroxides, peroxydicarbonates, dialkyl peroxides, peroxyesters, diperoxyesters, peroxyketals, hydroperoxides, 1,2-diphenylalkanes and hydrogen peroxide. It is also possible to use water-soluble initiators. Examples are sodium peroxodisulfate, potassium peroxodisulfate and ammonium peroxodisulfate. They are employed in the form of an aqueous solution. The proportion of water, based on the reaction mixture, should not be greater than 20% by weight. The water content is preferably less than 10% by weight.
The polymerization according to the invention is carried out as a solution polymerization in at least one hydrocarbon. Examples of hydrocarbons are all the abovementioned entraining agents. The entraining agent employed in the esterification is preferably used as solvent in the solution polymerization. Preferred solvents for the solution polymerization are toluene, xylene, cyclohexanone, methylcyclohexene and mixtures of aliphatic hydrocarbons having boiling points of from 70 to 160xc2x0 C. under atmospheric pressure. The process according to the invention is preferably carried out by metering a mixture of alkylpolyalkylene glycol esters and ethylenically unsaturated carboxylic acids and at the same time initiators and regulators at from 60 to 200xc2x0 C., preferably at from 70 to 150xc2x0 C., over the course of from 1 to 20 hours into a reactor in which the polymerization takes place. The reactor is usually rendered inert using nitrogen. When the metered addition is complete, the mixture is polymerized for a further 1 to 5 hours. Volatile components are then removed by distillation, and the polymer is diluted with water to the use concentration. However, it is also possible firstly to add the requisite amount of water in order to prepare from 50 to 80% strength by weight aqueous polymer solutions, and then to remove the solvent used in the polymerization from the mixture by azeotropic distillation. It is likewise possible firstly to remove only some of the solvent employed in the solution polymerization by distillation after the polymerization, then to add water and to remove the remainder of the solvent by azeotropic distillation.
In accordance with the process according to the invention, it is possible to prepare polymers having a molecular weight in the range from 15,000 to 65,000 in the form of 70 to 95% strength by weight solutions in hydrocarbons. This is advantageous in industry in order to achieve a high space-time yield of reactors.
The proportion of organic solvents can be up to 30% by weight during the polymerization. The at least 80% strength by weight polymer solutions are readily flowable and stirrable at temperatures above 60xc2x0 C. Since they have a high viscosity at below 50xc2x0 C., they are diluted to the use concentration with water, with the organic solvents being removed from the mixture by distillation before, during or after the addition of water. The solutions obtainable in this way can then readily be handled at ambient temperature. The service concentration of the aqueous solution is preferably from 50 to 80% by weight. The aqueous 50 to 80% by weight polymer solutions are readily flowable, pourable and pumpable. The viscosities are from 500 mPas to 10,000 mPas. A further advantage over the prior art is that the polymer solutions are free from foreign ions, for example contain no sodium sulfate. The molecular weight Mw of the polymers is from 15,000 to 65,000, preferably from 20,000 to 40,000. The free acid groups of the polymers are, if desired, neutralized using bases, preferably using aqueous sodium hydroxide solution.
The percentages in the examples are % by weight. The K value of the polymers was determined by the method of H. Fikentscher, Cellulose-chemie, Vol. 13, 58-64 and 71-74 (1932), in aqueous solution at a pH of 7, a temperature of 25xc2x0 C. and a polymer concentration of the sodium salt of the copolymers of 1% by weight.